The control of the cell cycle in early sea urchin and starfish embryos will be studied in terms of nucleo-cytoplasmic interactions. Aphidicolin, a recently discovered drug which inhibits DNA replication specifically, will be applied to early embryos in connection with the recently developed technique of egg fusion. This experimental approach is designed to make several new determinations about cell cycle control in the early embryo: (1) whether or not the unreplicated nucleus exerts negative control over progression through the cell cycle; (2) whether or not the cytoplasm of the replicated cell can exert positive control over the unreplicated nucleus; and (3) whether or not any controlling factors inferred from these studies can cross taxonomic class barriers between sea urchins and starfish. Additional new determinations about necleocytoplasmic interactions in unfertilized eggs and immature oocytes using the egg fusion technique only include: (4) whether or not nuclear DNA replication is stimulated in an unfertilized sea urchin egg upon fusion with a fertilized egg; (5) whether or not germinal vesicle breakdown and maturation of an immature sea urchin oocyte is stimulated upon fusion with an unfertilized egg; (6) whether or not germinal vesicle breakdown and maturation of an immature starfish oocyte is stimulated upon fusion with a mature starfish oocyte; and (7) whether or not any controlling factors inferred from these studies can cross taxonomic class barriers between sea urchins and starfish. These determinations should provide a more complete understanding of how the nucleus and cytoplasm are related with one another during the production of the egg and through early cleavage of the embryo.